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Foster–Seeley discriminator

The Foster–Seeley discriminator^[1][2] is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster^[3] and Stuart William Seeley. The Foster–Seeley discriminator was envisioned for automatic frequency controlofreceivers, but also found application in demodulating an FM signal.

The Foster–Seeley discriminator uses a tuned RF transformer to convert frequency changes into amplitude changes. A transformer, tuned to the carrier frequency, is connected to two rectifier diodes. The circuit resembles a full bridge rectifier.

The phase of the voltage at the secondary coil depends on whether the carrier is below or above the resonance, resulting in a positive or negative shift, respectively.

The circuit makes use of the ${\displaystyle 90^{\circ }}$ phase difference occurring between the voltages in two coupled resonant circuits at the peak frequency.

Through the coupling capacitor ${\displaystyle C_{k}}$, the primary voltage is applied to the center tap of the secondary, producing a sum and a difference ${\displaystyle V_{center}\pm {\frac {V_{secondary}}{2}}}$ on the top and the bottom terminals thereof, respectively.

Depending on the changing phase relationship of the input signal — ${\displaystyle V_{primary}}$ (and therefore ${\displaystyle V_{center}}$) with ${\displaystyle V_{secondary}}$, the amplitude of the sum/difference at the upper or lower half, respectively, will be higher, which results in a change of the voltage across the output capacitors. The choke ${\displaystyle L3}$, sometimes replaced by a resistor, provides a DC path through the rectifier diodes.

The demodulator's bandwidth depends on the Q factor of the resonant circuit; the phase response of the secondary (and therefore, the voltage response of the circuit) to ${\displaystyle \Delta {f}}$ is an S-curve.

Foster–Seeley discriminators are sensitive to both frequency and amplitude variations, unlike some detectors. Therefore a limiter amplifier stage must be used before the detector, to remove amplitude variations in the signal which would be detected as noise. The limiter acts as a class-A amplifier at lower amplitudes; at higher amplitudes it becomes a saturated amplifier which clips off the peaks and limits the amplitude.

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• Ratio detector
• Quadrature detector
• Phase-locked loop detector

• schematic and operation